Previous work by our lab (and by other groups) has determined that Mdm2 is the master regulator of the p53 tumor suppressor during development and in multiple tissues of neonates and adults. Mdm2 binds with p53 to block the p53's ability to alter transcription. In addition, Mdm2-p53 binding also promotes p53 ubiquitination, nuclear export, and proteosomal degradation, thereby inhibiting all p53 activities within the cell. Through these mechanisms, Mdm2 negatively regulates the ability of p53 to suppress tumorigenesis, and amplification and/or overexpression of the MDM2 gene is observed in a very significant percentage of human cancers. In this proposal, we summarize the progress we have made in our previous studies on p53 regulation by Mdm2 and its homologue, Mdm4 (MdmX). We describe recent studies examining the effects of ATM- mediated phosphorylation of Mdm2 on the regulation of p53 stability and activity, and discuss the generation of new Mdm2 alleles in mice that are compromised in their capacity to be phosphorylated by the DNA damage effector kinases ATM, c-Abl, or Akt. Based upon the preliminary data we present, we propose to utilize these models to examine in vivo the effects of Mdm2 phosphorylation on Mdm2-p53 signaling and on p53's ability to govern cell growth, the DNA damage response, cell metabolism, and to suppress tumorigenesis. As with our previous in vivo studies, this proposed research should greatly increase our understanding of the functional roles of the Mdm2-p53 signaling axis in regulating cell growth and death, and should help identify new targets for the treatment of radiation sickness and cancer.